Enoch Powell would have been 100 this year and Biteback has published a book to mark the anniversary. Its editor – Lord Howard of Rising (the Tory Greville Howard) – insists that Enoch at 100 was "never intended to be a hagiography", but rather a "critical assessment" of Powell's career.

Well, if this isn't a hagiography, it's hard to know what one might look like. From its livery (solemn portraits on the front cover and the frontispiece, an opening epigraph comparing Powell's influence to Churchill's) to the judgments of all but one of the articles, the book shrieks "tribute". Large tracts of it are by Powell himself: great slabs of direct quotation reappear in adjacent transcripts of Powell speeches. Even when venturing beyond quotation, contributors such as Frank Field pick up elements of Powell's quaint, circumlocutory prose style ("we can only conclude", "it must be conceded", "I happen to disagree"). From Nicholas True via Simon Heffer to Andrew Roberts, the book's contributors agree that, on matters ranging from defence and Europe to energy policy and the constitution, Enoch was right.

The question is, which Enoch? The Tory imperialist who advised Churchill how many divisions he'd need to retake India, or the Little Englander who argued that democratic states can't run empires? The frontbencher who voted for Britain's 1967 application to join Europe, or the backbencher who advised his supporters to vote Labour to get us out? The constitutionalist who thought devolved parliaments posed a fatal threat to a unitary legislature, or the Ulster Unionist who defended Stormont (until, again, he changed his mind)? And, indeed, the neo-liberal champion of free movement of capital, or the nationalist adversary of a free movement of labour?

To all this, his apologists would answer that Powell's mutating opinions were tactical responses to changing circumstances; underlying them was a core belief in a single, uncompromisable national identity, made manifest in "the crown in parliament" – a concept, for Roger Scruton, comparable to the real presence of Christ's body in the eucharist wafer. Hence, perhaps, the fact that – like those of the Roman Catholic church – Powell's flipflops don't feel like changes of mind, but rather moves from one state of certainty to another. Hence, too, the rhetorical authority of a man who believed himself inherently superior to all those around him. Powell's despair at "the almost unlimited capacity of my fellow countrymen for self-delusion" comes from a 1987 speech, but could have been made at any time in his career. Acknowledgement of previous error just goes to confirm how much cleverer than anyone else he remains. He is like Hercule Poirot, berating himself as an imbecile for not spotting in chapter three the vital element that will elude everyone else till the last page.

Which is, as often as not, the mystical oneness of – and treasonable threats to – a political unit which has not existed in the form he refers to it since 1701. Mushy English nativism dressed up as fierce logic informs, of course, Powell's notorious 1968 rivers of blood speech (or, as he would have preferred it, his "Thybrim multo spumantem sanguine" speech, the form in which he insists he declaimed its signature, Virgilian phrase). Powell's motivation for making it, and the credibility of its contents, is central to any genuine reappraisal of Powell's career.

Far and away the best in the book, Tom Bower's chapter does indeed ask why – despite silence throughout the decade during which most primary Commonwealth immigration occurred – Powell suddenly became interested in the question in the mid-1960s. For Paul Foot, in his 1969 The Rise of Enoch Powell, the answer is that Powell spotted the electoral potency of the issue in the West Midlands. For Bower, Powell was using immigration as a stick to beat Tory leader Edward Heath, who promptly sacked Powell from the shadow cabinet, before authorising a manifesto promising an end to primary new Commonwealth immigration and a programme of voluntary repatriation.

Bower challenges the other contributors' protestations that – however misjudged the speech – Powell was not a racist. Powell's insistence that the British-born offspring of new Commonwealth immigrants remain inherently alien (as Foot quotes it, "the West Indian or Asian does not by being born in England become an Englishman"), and that British institutions were endangered by people with "readily visible differences", were essentially racist positions. And Bower quotes a 1995 Powellism unavailable to Foot: "What's wrong with racism? Racism is the basis of nationality."

If the 1968 speech was indeed a piece of rabble-rousing opportunism, the credibility of Powell's evidence is as important as the erudition of his rhetoric. The most famous passage of the speech quotes a letter from a woman in Northumberland about a Wolverhampton widow whose street had – in Powell's charming phrase – "gone black". As a consequence, she said, her windows were broken, excreta was pushed though her letter box, and she was pursued down the street by "wide-grinning piccaninnies". Powell never claimed he knew or spoke to the Wolverhampton widow, and her and the letter's existence was questioned at the time. In something of a scoop in the current book, Powell's widow confirms that the letter existed, because she had seen it, but then lost it (probably, she says, putting it "somewhere so safe that we could never find it again").

There is however another explanation of what happened to the letter. In 2007, Powell's erstwhile secretary assured a BBC radio investigator that the letter was not lost, but kept in the inner sleeve of Powell's wallet. Further, the secretary knew the name of the widow, though he had pledged himself never to reveal it.

Of course, both explanations could be true. But surely, the existence and contents of the letter are important enough to be explored and challenged in a book which seeks critically to reappraise the career of Enoch Powell. Particularly as the secretary in question was Greville, now Lord, Howard, its editor.